1. Field of the Invention
The present invention relates to a wavelength conversion laser apparatus for converting wavelength by utilizing a nonlinear optical crystal, more specifically to a wavelength conversion laser apparatus in which the nonlinear optical crystal can be easily managed.
2. Description of Related Art
In short wavelength laser apparatus, nonlinear optical crystals are generally utilized for wavelength conversion. Ultra violet solid state laser devices among such laser apparatus are generally constructed as follows. A fundamental wave laser light of 1064 nm in wavelength is oscillated by a Nd:YAG laser, Nd:YVO4 laser or the like. The nonlinear optical crystal generates a second harmonic wave of the fundamental wave laser light. Furthermore, third and fourth harmonic waves are also generated by nonlinear optical crystals. Crystals to be utilized for generating the second harmonic wave include an LBO crystal or a KTP crystal. Crystals to be utilized for generating the third harmonic wave include an LBO crystal, a BBO crystal or GdYCOB crystal. Crystals to be utilized for generating the fourth harmonic wave include a BBO crystal, a CLBO crystal or the like. Most of such nonlinear optical crystals are deliquescent. In order to prevent them from degrading by moisture absorption, it is required to pay special attention to atmosphere where the crystals are disposed, especially humidity where they are used.
Particularly, a CLBO crystal which is the nonlinear optical crystal to be utilized for generating the fourth harmonic wave exhibits significant crystal degradation in 30% or higher relative humidity. A crystal cell model no. 10031 commercially available from Crystal Association Inc. employs a method for filling a dry gas in the cell. Alternatively, there is an instance where a nonlinear optical crystal is used in dipping into oil. In Japanese patent non-examined publication no. 9-292638 (or JP-A-292638/94) entitled “High Output Ultra Violet Laser Light Generation Apparatus”, a deliquescent-free protection film is formed on the ultra violet light output end surface of the nonlinear optical crystal.
However, one problem associated with such conventional technique of simply filling a dry gas in a crystal cell is a limited lifetime. In other words, if any defect may occur in the sealing structure during a long time use, humidity in the sealed container tends to change. If the laser is continuously used in high humidity condition inside the cell, the nonlinear optical crystal is increasingly degraded and damaged. This accompanies with significant decrease in wavelength conversion efficiency and thus significant decrease in the laser output.